The invention relates to a system for presteaming wood chips or the like. In the past, two different general types of presteaming vessels have been utilized in the pulp processing field, a horizontal screw-conveyor type of presteaming vessel for treatment over about 212.degree. F -- such as shown in U.S. Pat. No. 3,298,899 -- and vertical presteaming vessels -- one type of which is shown in U.S. Pat. Nos. 3,532,594 and 3,578,554. The general purpose of presteaming utilizing either type of conventional presteamer is to remove air from the chips to be treated, and to preheat them. Presteaming is important in the conservation of energy in the pulp processing system and in preparing the chips for absorption of the cooking chemicals.
According to the present invention, the removal of air from wood chips and the like during preheating is facilitated by maintaining the chips at about atmospheric pressure, or slightly below atmosphere pressure, during presteaming. Under these circumstances the provision of air pollution equipment is necessary, however, since some air must constantly be withdrawn from the presteaming vessel in order to hold it at or slightly below atmospheric pressure. This is in contradistinction to the prior art patents mentioned above wherein the gases from the presteaming chamber are only periodically vented when the pressure builds up to too high a level (see valve 21 in U.S. Pat. No. 3,298,899, valve 12 in U.S. Pat. No. 3,532,594 and line 39 in U.S. Pat. No. 3,578,554). Economical operation of a vertical presteamer at or slightly below atmospheric pressure, because of the necessary air pollution equipment, depends upon not drawing too large an amount of gas through the presteaming vessel for treatment, since the cost and efficiency of the system is greatly dependent upon the amount of air treated.
According to the present invention, chips have the air removed therefrom in a presteaming vessel at or slightly below atmospheric pressure with a minimum amount of air removal from the presteamer itself. The top of the vertical presteaming vessel according to the present invention is closed and has a fan in operative communication therewith. The fan essentially continuously operates and removes gas from the vessel and transports it to a condenser and further air pollution control equipment. The chips are fed into the vessel by a rotating inclined screw with a partial flight. The flight pushes a plug of chips into the vessel, the chip plug providing an effective air lock, and preventing the movement of too large a volume of air through the chips under the suction of the fan. Since the air lock is provided by the chips and the like themselves, and there are no closely rotating metal or other material parts to form the isolation seal, the wear on the feeder is minimal. A temperature control is provided in the exhaust from the fan to control the amount of low pressure steam that is introduced into the presteaming vessel (the invention is useful for treatment at about 180.degree.-190.degree. F), and a pressure control is provided between the fan (or a valve associated therewith) and the interior of the presteaming vessel to control the amount of gas exhausted from the presteaming vessel to keep the vessel at the desired pressure at or slightly below atmospheric.
It is the primary object of the present invention to provide for improved pretreatment of wood chips in a vertical presteaming vessel. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.